Release of Bound Trypsin Inhibitors in Soybeans by Rhizopus oligosporus

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Abstract
Activity of soybean trypsin inhibitors (SBTI) of boiled soybeans fermented by Rhizopus oligosporus Saito was studied to establish their nature. Extracts prepared from fermented boiled soybeans showed higher trypsin-inhibiting activity than extracts prepared from boiled soybeans. The extractable SBTI increased as the fermentation progressed, and reached a maximum after 48 hours of incubation; thereafter, SBTI activity decreased. The increased trypsin inhibitor was not synthesized by the mold, because no inhibitory activity was found in the culture filtrates of mold grown in milk or wheat media. Increase of trypsin-inhibitory activity was also noted when heated soybeans were treated with partially purified extracellular proteases by R. oligosporus. It is, therefore, apparent that an active trypsin inhibitor was liberated from a heat-resistant, inactive, bound form by R. oligosporus proteases. Once released, the inhibitor was readily inactivated by heat. Rhizopus oligosporus protease was also found to inactivate crystalline SBTI. Therefore, SBTI activity observed in boiled soybeans either fermented by R. oligosporus or treated with R. oligosporus enzymes came from released SBTI and from SBTI not inactivated by the mold enzymes.